The use of permanent magnet rotors in generators has created a problem with voltage control as the generator load varies. The output voltage of a dual permanent magnet generator is controlled by shifting the two rotors in and out of phase. For example, at a two per unit load, the magnetic poles would be lined up, while at no load, the poles would be 78.degree. out of phase.
An alternating current generator having a twin permanent magnet rotor which is adjustable in response to output voltage is known from U.S. Pat. No. 3,713,015. As disclosed therein, only one rotor is actuated using a lever to force sliding motion for achieving a relative rotation of the rotors. This known arrangement involves considerable friction and therefor requires significant power for overcoming inertia to angularly adjust the rotors.